Imaging systems for obtaining spectral data of a plurality of inspection points of an inspected surface are widely used for automatically inspecting various objects in a great variety of applications such as automated recycling of plastic and other materials for example. They generally comprise a lighting unit for projecting light onto the objects conveyed and an imaging unit for imaging the light reflected from the objects, as illustrated in the imaging system 100 of FIG. 1 (PRIOR ART). The reflected light may then be analyzed to characterize the objects before sorting them.
Such imaging systems devised to automatic material sorting are described in U.S. Pat. No. 7,113,272 and in PCT application publication numbers WO2004016362; WO9606689; WO0057160 and WO2005106438.
Each of these references discloses an imaging system minimizing or eliminating specular reflections which are described as parasite reflections. They rely on diffused reflected light to characterize the objects.
Although these systems may be generally suitable for inspection of objects having diffusing surfaces, they may not be adapted for inspecting transparent and/or light material having poorly diffusing surfaces, which is of great disadvantage.
Indeed, in the prior art systems, a small quantity of diffused light is nevertheless reflected by the poorly diffusing surfaces but the measured signal is generally too small to provide accurate information. Moreover, the measured signal may even be lost in the noise in some cases.
In order to overcome this disadvantage, it has been proposed to rely on diffused light reflected by a surface located under the transparent object and which has traveled through the object under inspection.
In the case of automatic material sorting, a conveying belt is used for conveying the objects under the imaging system and then to a sorting station. The diffused light reflected by the belt may then be used. However, in many applications, nominal conditions of the belt may change over time and prevent a reliable inspection of the objects, which is of great disadvantage.
For example, in the case the belt is clean, his surface may be highly specular, thereby preventing diffused reflections towards the imaging unit. Alternatively, in the case the belt is dirty, diffused reflections towards the imaging unit may be enabled, but these reflections may not be considered as enough reliable for some applications since they may be altered by the spectral signature of the conveying belt.
Moreover, even if the proposed configurations help minimizing the specular reflections, such specular reflections may nevertheless reach the imaging unit and saturate the imaging element. In order to reduce this issue, the imaging element may be provided with a high dynamic range in order to minimize saturation issues while enabling maximum detection of diffused light, but saturation issues may still arise.
Other configurations wherein each of the imaging unit and the lighting unit extends on both side of the matter respectively have been proposed for imaging light passing through transparent objects, but they may not be conveniently and cost-effectively implemented in an online industrial process, particularly in dirty environments, which is of great disadvantage.
It would therefore be desirable to provide an improved apparatus and an improved method for inspecting various objects that would reduce at least one of the abovementioned drawbacks.